Nonaqueous electrolytic solution and lithium secondary battery

ABSTRACT

A nonaqueous electrolytic solution for lithium secondary batteries. When the nonaqueous solvent comprises a combination of an ester of a tertiary carboxylic acid and a cyclic carbonate such as propylene carbonate or ethylene carbonate, a lithium salt having a fluorine atom is preferably used as the electrolyte salt. In this case, the ester of a tertiary carboxylic acid is preferably used in a relatively small amount, especially in an amount of about 0.5 to 35 wt. % based on the nonaqueous solvent.

FIELD OF INVENTION

The present invention relates to a non-aqueous electrolytic solutionwhich imparts to lithium secondary batteries excellent batterycharacteristics in battery cycle property, electric capacity and storageproperty and further relates to a lithium secondary battery containingthe same.

BACKGROUND OF INVENTION

At present, a lithium secondary battery is generally employed as anelectric source for driving small electronic devices. The lithiumsecondary battery essentially comprises a positive electrode, anon-aqueous electrolytic solution, separator, and a negative electrode.A lithium secondary battery utilizing a positive electrode of lithiumcompound oxide such as LiCoO₂ and a negative electrode of carbonaceousmaterial or lithium metal is preferably used. As the electrolyticsolution, a cyclic carbonate such as ethylene carbonate (EC) orpropylene carbonate (PC) is preferably used.

Under the conditions, Japanese Patent Provisional Publication 7-37613provides an electrolytic solution for lithium secondary batteries thatemploys an electrolytic solution comprising a combination of a commonnon-aqueous solvent (e.g., EC, PC) and a non-aqueous tertiary carboxylicester solvent such as methyl trifluoroacetate (MTFA) or methyl pivalate(MPA), is stable in a voltage range higher than 4 V, and shows highelectroconductivity in a temperature range of 0° C. or lower, lowreactivity with lithium and long charge-discharge cycle life.

In the Japanese Patent Provisional Publication 7-37613, there aredescribed a negative electrode of glassy carbon and a non-aqueoussolvent of a mixture of PC and MTFA, a mixture of EC and MTFA, and amixture of PC and MPA.

According to the study of the present inventors, however, it has beenfound that when generally employed carbonaceous material such as naturalgraphite or artificial graphite, especially highly crystallizedcarbonaceous material such as highly crystallized natural graphite orartificial graphite, is employed for the formation of a negativeelectrode in a lithium secondary battery, an electrolytic solutionemploying the above-mentioned non-aqueous mixture solvent is apt todecompose on the negative electrode to cause increase of irreversiblecapacity, and further sometimes causes exfoliation of carbonaceousmaterials. The increase of irreversible capacity and exfoliation ofcarbonaceous material are caused by decomposition of the solvent in theelectrolytic solution at the time of charging and by electrochemicalreduction of the solvent on the interface between the carbonaceousmaterial and the electrolytic solution. Particularly, PC (propylenecarbonate) having a low melting point and a high permittivity andshowing a high electroconductivity even at a low temperature decomposesin the presence of a graphite negative electrode and therefore it cannotbe used for a lithium secondary battery. Further, it has been found thatEC (ethylene carbonate) also decomposes in portion in the course ofrepeated charge-discharge procedures so that the battery characteristicslower. It has been furthermore found that since methyl pivalate has aboiling point of 101° C., a non-aqueous solvent comprising 50 wt. % ormore of methyl pivalate causes expansion of the battery and lowering ofbattery characteristics at elevated temperatures.

It is also found that the use of an electrolytic salt of LiClO₄ (whichis described in the Japanese Patent Provisional Publication 7-37613) inthe electrolytic solution causes decomposition of the electrolyticsolution to emit gas in the operation of battery at a high temperatureand gives harmful effect to cycle property at 0° C. or higher.

Japanese Patent Provisional Publication 12-182670 describes a lithiumbattery which utilizes as the non-aqueous solution such a compound asethyl acetate, methyl propionate or methyl butyrate and is employable atlow temperatures. The non-aqueous battery described in the JapanesePatent Provisional Publication 12-182670 certainly shows goodcharacteristics at low temperatures. However, a carboxylic acid esterhaving a structure in which a hydrogen atom is attached to a carbon atomplaced adjacent to a carbonyl group, such as ethyl acetate, methylpropionate or methyl butyrate, is apt to produce a gas upon reactionwith lithium metal (which is produced on the negative electrode byelectrodeposition) and causes lowering of cycle property and lowering ofstorage endurance, in the case that it is employed in combination with anegative electrode of highly crystallized carbonaceous material such asnatural graphite or artificial graphite in a lithium secondary battery.Further, in the case of employing a non-aqueous solvent containing 20wt. % or more of methyl propionate (boiling temperature: 79° C.),expansion of the battery and lowering of the battery characteristics areobserved.

Japanese Patent Provisional Publication 9-27328 describes that anon-aqueous electrolytic solution containing methyl decanoate, dodecylacetate, or the like easily permeates into a separator, and that thenon-aqueous battery utilizing such non-aqueous electrolytic solutionshows large battery capacity and high battery voltage, and further showslittle fluctuation of battery characteristics. The non-aqueous batterydescribed in the Japanese Patent Provisional Publication 9-27328certainly shows good permeation into an electrolytic solution into aseparator, large battery capacity and high battery voltage. However, inthe case that the disclosed electrolytic solution is used in combinationwith highly crystallized carbonaceous material such as natural graphiteor artificial graphite in lithium secondary batteries, the solution maydecompose on the negative electrode to cause increase of irreversiblecapacity and sometimes causes exfoliation of carbonaceous material. Theincrease of irreversible capacity and exfoliation of carbonaceousmaterial are caused by decomposition of the solvent in the electrolyticsolution in the charging process and originate from electrochemicalreduction of the solvent on the interface between the carbonaceousmaterial and the electrolytic solution. Particularly, a carboxylic acidester having a structure in which a hydrogen atom is attached to acarbon atom placed adjacent to a carbonyl group, such as methyldecanoate or dodecyl acetate, is apt to decompose in portion in thecourse of the repeated charging-discharging procedures and causeslowering of cycle property in the case that the graphite negativeelectrode is employed.

DISCLOSURE OF INVENTION

It is an object of the present invention to solve the problems observedin the heretofore known electrolytic solutions for lithium secondarybatteries and provide a lithium secondary battery which shows goodbattery cycle property, good electric capacity, good storage endurancein the charged conditions, and further shows little battery expansion inthe use at elevated temperatures. The invention further provides anon-aqueous electrolytic solution favorably employable for lithiumsecondary batteries.

The present inventors have discovered that a fluorine-containing lithiumsalt is preferably employed as the electrolytic salt when the tertiarycarboxylic ester described in the aforementioned Japanese PatentProvisional Publication 7-37613 is employed in combination with a cycliccarbonate such as propylene carbonate or ethylene carbonate. They havefurther discovered that, in the above-mentioned case, the tertiarycarboxylic ester is preferably employed in a relatively small amount,particularly in an amount of 0.5 to 35 weight % in the non-aqueoussolvent.

The inventors have furthermore discovered that a tertiary carboxylicacid ester containing an alcohol reside of an alkyl group having 4 ormore carbon atoms is highly compatible with a separator comprisingpolyolefin such as polypropylene or polyethylene and therefore it isfavorably employed as a component of non-aqueous solvent for lithiumsecondary batteries.

Accordingly, the invention resides in a lithium secondary battery whichcomprises a positive electrode comprising lithium compound oxide, anegative electrode comprising carbonaceous material, a separator, and anon-aqueous electrolytic solution comprising an electrolytic salt in anon-aqueous solvent, in which the electrolytic salt is afluorine-containing lithium salt and the non-aqueous solvent contains acyclic carbonate and 0.5 to 35 weight % of a tertiary carboxylic esterhaving the formula (I):

[wherein each of R¹, R² and R³ independently represents methyl, ethyl,fluorine, or chlorine, and R⁴ represents a hydrocarbyl group having 1-20carbon atoms].

The invention further resides in a lithium secondary battery whichcomprises a positive electrode comprising lithium compound oxide, anegative electrode comprising carbonaceous material, a separator, and anon-aqueous electrolytic solution comprising an electrolytic salt in anon-aqueous solvent, in which the electrolytic salt is afluorine-containing lithium salt and the non-aqueous solvent contains acyclic carbonate and 0.5 weight % or more of a tertiary carboxylic esterhaving the above-mentioned formula (I) [wherein each of R¹, R² and R³independently represents methyl, ethyl, fluorine, or chlorine, and R⁴represents a hydrocarbyl group having 4-20 carbon atoms].

Furthermore, the invention resides in a non-aqueous electrolyticsolution for lithium secondary batteries comprising an electrolytic saltin a non-aqueous solvent, in which the electrolytic salt is afluorine-containing lithium salt and the non-aqueous solvent contains acyclic carbonate and 0.5 to 35 weight % of a tertiary carboxylic esterhaving the above-mentioned formula (I) [wherein each of R¹, R² and R³independently represents methyl, ethyl, fluorine, or chlorine, and R⁴represents a hydrocarbyl group having 1-20 carbon atoms].

Furthermore, the invention resides in a non-aqueous electrolyticsolution comprising an electrolytic salt in a non-aqueous solvent, inwhich the electrolytic salt is a fluorine-containing lithium salt andthe non-aqueous solvent contains a cyclic carbonate and 0.5 weight % ormore of a tertiary carboxylic ester having the above-mentioned formula(I) [wherein each of R¹, R² and R³ independently represents methyl,ethyl, fluorine, or chlorine, and R⁴ represents a hydrocarbyl grouphaving 4-20 carbon atoms].

DETAILED DESCRIPTION OF INVENTION

In the aforementioned formula (I) for the tertiary carboxylic acid esterto be incorporated into an electrolytic solution in which anelectrolytic salt is dissolved in a non-aqueous solvent, it is preferredthat each of R¹, R² and R³ independently is methyl or ethyl. R⁴preferably is an alkyl group having 1-20 carbon atoms such as methyl,ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl,undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl,heptadecyl, octadecyl, or eicosanyl. The alkyl group may be a branchedalkyl group such as isopropyl, isobutyl, isopentyl, sec-butyl,tert-butyl, isooctyl, sec-octyl, 2-ethylhexyl, isononyl, isodecyl, orisooctadecyl. Further, an unsaturated hydrocarbyl group such as vinyl,allyl, or propargyl can be employed. Furthermore, an aryl group such asphenyl, tolyl or biphenyl, or a benzyl group can be employed.

Examples of the tertiary carboxylic esters of the formula (I) includemethyl pivalate (R¹=R²=R³=R⁴=methyl), ethyl pivalate. (R¹=R²=R³=methyl,R⁴=ethyl), propyl pivalate (R¹=R²=R³=methyl, R⁴=n-propyl), isopropylpivalate (R¹=R²=R³=methyl, R⁴=isopropyl), butyl pivalate(R¹=R²=R³-methyl, R⁴=n-butyl), sec-butyl pivalate (R¹=R²=R³=methyl,R⁴=sec-butyl), isobutyl pivalate (R¹=R²=R³=methyl, R⁴=isobutyl),tert-butyl pivalate (R¹=R²=R³=methyl, R⁴=tert-butyl), octyl pivalate(R¹=R²=R³=methyl, R⁴=n-octyl), sec-octyl pivalate (R¹=R²=R³=methyl,R⁴=sec-octyl), nonyl pivalate (R¹=R²=R³=methyl, R⁴=n-nonyl), decylpivalate (R¹=R²=R³=methyl, R⁴=n-decyl), undecyl pivalate(R¹=R²=R³=methyl, R⁴=n-undecyl), dodecyl pivalate (R¹=R²=R³=methyl,R⁴=n-dodecyl), vinyl pivalate (R¹=R²=R³=methyl, R⁴=vinyl), allylpivalate (R¹=R²=R³=methyl, R⁴=allyl), propargyl pivalate(R¹=R²=R³=methyl, R⁴=propargyl), phenyl pivalate (R¹=R²=R³=methyl,R⁴=phenyl), p-tolyl pivalate (R¹=R²=R³=methyl, R⁴=p-tolyl), biphenylpivalate (R¹=R²=R³=methyl, R⁴=biphenyl), benzyl pivalate(R¹=R²=R³=methyl, R⁴=benzyl), methyl 2,2-dimethylbutanoate(R¹=R²=R⁴=methyl, R³=ethyl), methyl 2-ethyl-2-methylbutanoate(R¹=R⁴=methyl, R²=R³=ethyl), and methyl 2,2-diethylbutanoate(R¹=R²=R³=ethyl, R⁴=methyl).

The tertiary carboxylic ester employed in the invention is not limitedto the above-mentioned examples. Various combinations can be employed inconsideration of the gist of the invention.

As is described hereinbefore, if the tertiary carboxylic ester of theformula (I) is contained in the electrolytic solution in an excessivelylarge amount, the electroconductivity of the electrolytic solution mayvary, while if it is contained in an extremely small amount, theexpected battery characteristics can be obtained. Accordingly, theamount of the tertiary carboxylic ester preferably is in the range of0.5 to 35 weight %, particularly in the range of 1 to 20 weight %.

The non-aqueous solvent used in the invention necessarily contains thetertiary carboxylic ester and a cyclic carbonate. It is preferred that alinear carbonate is added to the solvent.

Examples of the cyclic carbonates include ethylene carbonate (EC),propylene carbonate (PC), butylene carbonate (BC), and vinylenecarbonate (VC). These cyclic carbonates can be used singly or incombination.

Examples of the linear carbonates include dimethyl carbonate (DMC),diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propylcarbonate (MPC), isopropyl methyl carbonate (IPMC), butyl methylcarbonate (BMC), isobutyl methyl carbonate (IBMC), sec-butyl methylcarbonate (SBMC), and tert-butyl methyl carbonate (TMBC). These linearcarbonates can be used singly or in combination.

An optionally selected cyclic carbonate and an optionally selectedlinear carbonate can be employed in combination. The non-aqueous solventpreferably contains a cyclic carbonate in an amount of 10 to 80 weight%. If a linear carbonate is employed in combination, its amountpreferably is not more than 80 weight %.

The non-aqueous solvent of the invention can further contain a cyclicester. Preferred examples of the cyclic esters include γ-butyrolactone(GEL) and γ-valerolactone (GVL). The cyclic esters can be employedsingly or in combination. If the cyclic ester is contained in thenon-aqueous solvent, its amount preferably is not more than 70 weight %,particularly in the range of 30 to 70 weight %. Since the cyclic esterhas a high flash point, the incorporation of the cyclic ester into anelectrolytic solution enhances safety of a lithium secondary battery.

The electrolytic salt to be incorporated into the electrolytic solutionof the invention is a fluorine atom-containing lithium salt. Preferredexamples of the fluorine-containing lithium salts include LiPF₆, LiBF₄,LiAsF₆, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiC(SO₂CF₃)₃, LiPF₄(CF₃)₂,LiPF₃(C₂F₅)₃, LiPF₃(CF₃)₃, LiPF₃(iso-C₃F₇)₃, and LiPF₅(iso-C₃F₇). Theseelectrolytes can be employed singly or in combination. Generally, theelectrolyte can be incorporated into the non-aqueous solvent in such anamount to give an electrolytic solution of 0.1 M to 3 M, preferably 0.5M to 2 M.

The electrolytic solution can be prepared, for instance, by mixing theabove-mentioned cyclic carbonate and linear carbonate (if desired, acyclic ester is further mixed); dissolving a tertiary carboxylic esterof the formula (I) in the mixture; and further dissolving theaforementioned electrolytic fluorine-containing lithium salt in themixture solution.

The electrolytic solution of the invention is favorably employable formanufacture of lithium secondary battery.

The positive electrode to be employed in a lithium secondary batteryaccording to the invention comprises a lithium compound metal oxide.Positive electrode materials (active materials of positive electrode)are compound oxides of lithium and at least one metal selected from thegroup consisting of cobalt, manganese, nickel, chromium, iron, andvanadium. Examples of the compound metal oxides include LiCoO₂, LiMn₂O₄,and LiNiO₂. Further, a compound metal oxide of lithium with a mixture ofcobalt and manganese and a compound metal oxide of lithium with amixture of cobalt and nickel are employable.

The positive electrode can be manufactured by kneading theabove-mentioned positive material, an electro-conductive material suchas acetylene black or carbon black, and a binder such aspoly(tetrafluoroethylene) (PTFE), poly(vinylidene fluoride) (PVDF),styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer(NBR) or carboxymethylcellulose (CMC) to give a positive electrodecomposition; coating the positive electrode composition on a collectorsuch as aluminum foil, stainless foil, or lath plate; drying the coatedcomposition; pressing the dried composition; and heating the pressedcomposition in vacuo at a temperature of approximately 50 to 250° C. forapproximately 2 hours.

As the negative electrode active material, carbonaceous material havinga graphite crystal structure (e.g., thermally decomposed carbonaceousmaterial, coke, graphites such as artificial graphite and naturalgraphite, fired organic polymer, and carbon fiber) which can absorb andrelease lithium. It is preferred to employ carbonaceous materials havinga graphite crystal structure in which the lattice distance of latticesurface (002), namely, d₀₀₂, is in the range of 0.335 to 0.340 nm(nanometer). The negative electrode active material in the powdery formsuch as carbonaceous powder is preferably used in combination with abinder such as ethylene propylene diene terpolymer (EPDM),polytetrafluoro-ethylene (PTFE), poly(vinylidene fluoride) (PVDF),styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer(NBR) or carboxymethylcellulose (CMC).

Preferred separators are microporous separators of polyethylene such aspolypropylene and polyethylene. Separators made of other materials suchas woven fabrics and non-woven fabrics are also employable

There are no specific limitations with respect to the structure of thenon-aqueous secondary battery of the invention. For instance, thenon-aqueous secondary battery can be a battery of coin type comprising apositive electrode, a negative electrode, and single or pluralseparators, or a cylindrical or prismatic battery comprising a positiveelectrode, a negative electrode, and a separator roll.

The examples and comparison examples are given below.

EXAMPLE 1

-   1) Preparation of Non-aqueous Electrolytic Solution

In a non-aqueous mixture of EC (ethylene carbonate), DEC (diethylcarbonate) and methyl pivalate (compound of the formula (I) in whicheach of R¹, R², R³, and R⁴ is methyl) [weight ratio 30:60:10] wasdissolved LiPF₆ to give a non-aqueous electrolytic solution of 1Mconcentration.

-   2) Manufacture Lithium Secondary Battery and Measurement of its    Battery Characteristics

LiCoO₂ (positive electrode active material, 80 wt. %), acetylene black(electro-conductive material, 10 wt. %), and poly(vinylidene fluoride)(binder, 10 wt. %) were mixed. To the resulting mixture was furtheradded 1-methyl-2-pyrrolidone. Thus produced mixture was coated onaluminum foil, dried, pressed, and heated to give a positive electrode.

Artificial graphite (negative electrode active material, 90 wt. %) andpoly(vinylidene fluoride) (binder, 10 wt. %) were mixed. To theresulting mixture was further added 1-methyl-2-pyrrolidone. Thusproduced mixture was coated on copper foil, dried, pressed, and heatedto give a negative electrode.

The positive and negative electrodes, a microporous polypropylene filmseparator, and the above-mentioned non-aqueous electrolytic solutionwere combined to give a coin-type battery (diameter: 20 mm, thickness:3.2 mm).

The coin-type battery was charged at roan temperature (20° C.) with aconstant electric current (0.8 mA) to reach 4.2 V for 5 hours.Subsequently, the battery was discharged to give a constant electriccurrent (0.8 mA) to give a terminal voltage of 2.7 V. Thecharge-discharge cycle was repeated.

The initial discharge capacity was almost the same as the capacitymeasured in a battery using an 1M LiPF₄ and EC/DEC (30/70, weight ratio)solvent mixture (containing no additive) [see Comparison Example 3].

After the 50 cycle charge-discharge procedure, the retention ofdischarge capacity was 90.7% of the initial discharge capacity (100%).Further, various characteristics at low temperatures and hightemperatures were satisfactory.

The manufacturing conditions and the battery characteristics are shownin Table 1.

EXAMPLES 2 TO 11

The procedures of Example 1 were repeated except for varying theconditions as set forth in Table 1, to manufacture coin batteries. Themanufactured coin batteries were subjected to measurement of retentionof discharge capacity after 50 cycles. The results are set forth inTable 1.

COMPARISON EXAMPLES 1 TO 5

The procedures of Example 1 were repeated except for varying theconditions as set forth in Table 1, to manufacture coin batteries. Themanufactured coin batteries were subjected to measurement of retentionof discharge capacity after 50 cycles. The results are set forth inTable 1.

EXAMPLE 12

The procedures of Example 1 were repeated except for employing anon-aqueous solvent of EC/PC (propylene carbonate)/DEC/octyl pivalate(35/35/25/5, weight ratio) and a negative electrode of coke, tomanufacture a coin battery. The manufactured coin battery was subjectedto measurement of retention of discharge capacity after 50 cycles. Theretention of discharge capacity was 86.5% of the initial dischargecapacity (100%).

The manufacturing conditions and the battery characteristics are shownin Table 1.

COMPARISON EXAMPLE 6

The procedures of Example 12 were repeated except for employing anon-aqueous solvent of EC/PC/methyl octanoate (49/49/2, weight ratio),to manufacture a coin battery. The manufactured coin battery wassubjected to measurement of retention of discharge capacity after 50cycles. The retention of discharge capacity was 71.3% of the initialdischarge capacity (100%).

The manufacturing conditions and the battery characteristics are shownin Table 1.

EXAMPLE 13

The procedures of Example 1 were repeated except for using a non-aqueoussolvent of EC/GBL (γ-butyrolactone)/IBMC (isobutyl methylcarbonate)/DEC/octyl pivalate (25/50/20/5, weight ratio) in which 1.2MLiBF₄ and a positive electrode of LiMn₂O₄, to manufacture a coinbattery. The manufactured coin battery was subjected to measurement ofretention of discharge capacity after 50 cycles.

The retention of discharge capacity was 83.4% of the initial dischargecapacity (100%).

The manufacturing conditions and the battery characteristics are shownin Table 1.

EXAMPLE 14

The procedures of Example 13 were repeated except for employing anon-aqueous solvent of EC/GBL/IBMC/decyl pivalate (25/50/20/5, weightratio), to manufacture a coin battery. The manufactured coin battery wassubjected to measurement of retention of discharge capacity after 50cycles. The retention of discharge capacity was 82.1% of the initialdischarge capacity (100%).

The manufacturing conditions and the battery characteristics are shownin Table 1.

EXAMPLE 15

The procedures of Example 13 were repeated except for employing anon-aqueous solvent of EC/GBL/IBMC/dodecyl pivalate (25/50/20/5, weightratio), to manufacture a coin battery. The manufactured coin battery wassubjected to measurement of retention of discharge capacity after 50cycles. The retention of discharge capacity was 81.7% of the initialdischarge capacity (100%).

The manufacturing conditions and the battery characteristics are shownin Table 1.

COMPARISON EXAMPLE 7

The procedures of Example 13 were repeated except for employing anon-aqueous solvent of EC/GEL (30/70, weight ratio), to manufacture acoin battery. The manufactured coin battery was subjected to measurementof retention of discharge capacity after 50 cycles. The retention ofdischarge capacity was 67.4% of the initial discharge capacity (100%4).

The manufacturing conditions and the battery characteristics are shownin Table 1.

TABLE 1 Posi. Nega. Li salt Cyclic Linear Additive Retent. Example Elec.Elec. (M) (wt. %) (wt. %) (wt. %) (%)  1 LiCoO₂ Art. LiPF₆ EC DECM.pivalate 90.7 1M 30 60 10  2 LiCoO₂ Art. LiPF₆ EC DEC M.pivalate 91.81M 30 50 20  3 LiCoO₂ Art. LiPF₆ EC DEC M.pivalate 91.4 1M 30 40 30  4LiCoO₂ Art. LiPF₆ EC DEC M.pivalate 85.9 1M 30 30 40  1 LiCoO₂ Art.LiClO₄ EC None M.pivalate 65.5 (Com.) 1M 50 50  2 LiCoO₂ Art. LiClO₄ PCNone M.pivalate Fail (Com.) 1M 50 50  3 LiCoO₂ Art. LiPF₆ EC DEC None81.7 (Com.) 1M 30 70  5 LiCoO₂ Art. LiPF₆ EC/PC DEC E.pivalate 91.6 1M30/5 50 15  6 LiCoO₂ Art. LiPF₆ EC/PC/VC DMC/EMC M.pivalate 92.1 1M27/5/3 15/40 10  7 LiCoO₂ Art. LiPF₆ EC/PC/VC DMC/IPMC M.pivalate 91.927/5/3 15/40 10  8 LiMn₂O₄ Art. LiPF₆ EC/PC DEC M.pivalate 92.1 30/5 5015  4 LiCoO₂ Art. LiPF₆ EC/VC DMC E.acetate 81.1 (Com.) 1M 15/5 23 57  9LiCoO₂ Art. LiPF₆/ EC/PC/VC DEC M.pivalate 91.3 LiBF₄ 27/5/3 45 200.9M/0.1M 10 LaCoO₂ Art. LiPF₆/ EC/PC/VC DEC M.pivalate 91.6LiN(SO₂CF₃)₂ 27/5/3 45 20 0.9M/0.1M 11 LiCoO₂ Art. LiPF₆ EC/PC/VC DECO.pivalate 90.4 35/35/5 20 5  5 LiCoO₂ Art. LiPF₆ EC/PC None M.octanoateFail (Com.) 1M 49/49 2 12 LiCoO₂ Coke LiPF₆ EC/PC DEC O.pivalate 86.5 1M35/35 25 5  6 LiCoO₂ Coke LiPF₆ EC/PC None M.octanoate 71.3 (Com.) 1M49/49 2 13 LiMn₂O₄ Art. LiBF₄ EC/GBL IBMC O.pivalate 83.4 1.2M 25/50 205 14 LiMn₂O₄ Art. LiBF₄ EC/GBL IBMC D.pivalate 82.1 1.2M 25/50 20 5 15LiMn₂O₄ Art. LiBF₄ EC/GBL IBMC DD.pivalate 81.7 1.2M 25/50 20 5  7LiMn₂O₄ Art. LiBF₄ EC/GBL None None 67.4 (com.) 1.2M 30/70 Remarks:Art.: Artificial graphite, Cyclic: Cyclic carbonate or cyclic esterLinear: Linear carbonate M.pivalate: Methyl pivalate E.pivalate: Ethylpivalate O.pivalate: Octyl pivalate D.pivalate: Decyl pivalateDD.pivalate: Dodecyl pivalate Additive: Tertiary carbonic ester or otheresters Retent.: Retention of discharge capacity after 50 cycles Fail:Failure of charge-discharge cycle

From the results set forth in Table 1, it is understood that a lithiumsecondary battery employing a positive electrode of lithium compoundoxide material, a negative electrode of carbonaceous material, and anelectrolytic salt of a fluorine-containing lithium salt shows a highdischarge capacity retention when such a small amount as 35 weight % orless of a tertiary carboxylic ester of the formula (I), particularlypivalic ester, is added to a non-aqueous solvent comprising a cycliccarbonate and a linear carbonate.

COMPARISON EXAMPLE 8

The procedures of Example 1 were repeated except for employing anon-aqueous solvent of plural cyclic carbonates (EC/PC/VC) containing noa linear carbonate, to manufacture a coin battery. The manufactured coinbattery was subjected to measurements of an initial discharge capacityand a retention of discharge capacity after 50 cycles. The initialdischarge capacity is 0.45 (relative value which is obtained when theinitial discharge capacity in the aforementioned Comparison Example 3 isset to 1). The retention of discharge capacity was 15.2% of the initialdischarge capacity (100%). The manufacturing conditions and the batterycharacteristics are shown in Table 2.

EXAMPLE 16

The procedures of Example 1 were repeated except for employing anon-aqueous solvent of a mixture of plural cyclic carbonates and pivalicester (EC/PC/VC/methyl pivalate=45/45/5/5, weight ratio) containing no alinear carbonate, to manufacture a coin battery. The manufactured coinbattery was subjected to measurements of an initial discharge capacityand a retention of discharge capacity after 50 cycles. The initialdischarge capacity is 0.91 (relative value which is obtained when theinitial discharge capacity in the aforementioned Comparison Example 3 isset to 1). The retention of discharge capacity was 89.1% of the initialdischarge capacity (100%). The manufacturing conditions and the batterycharacteristics are shown in Table 2.

EXAMPLES 17 TO 21

The procedures of Example 16 were repeated except for replacing methylpivalate with ethyl pivalate (Example 17), butyl pivalate (Example 18),hexyl pivalate (Example 19), octyl pivalate (Example 20), decyl pivalate(Example 21), or dodecyl pivalate (Example 22), to manufacture a coinbattery. The manufactured coin batteries were subjected to measurementsof an initial discharge capacity and a retention of discharge capacityafter 50 cycles. The results are shown in Table 2.

TABLE 2 Posi. Nega. Li salt Cyclic Additive Retent. Example Elec. Elec.(M) (wt. %) (wt. %) Initial (%) 16 LiCoO₂ Art. LiPF₆ EC/PC/VC M.pivalate0.91 89.1 1M 45/45/5 5 17 LiCoO₂ Art. LiPF₆ EC/PC/VC E.pivalate 0.9589.5 1M 45/45/5 5 18 LiCoO₂ Art. LiPF₆ EC/PC/VC B.pivalate 1.00 90.4 1M45/45/5 5 19 LiCoO₂ Art. LiPF₆ EC/PC/VC H.pivalate 1.01 90.8 1M 45/45/55 20 LiCoO₂ Art. LiPF₆ EC/PC/VC O.pivalate 1.00 90.5 1M 45/45/5 5 21LiCoO₂ Art. LiPF₆ EC/PC/VC D.pivalate 1.00 90.4 1M 45/45/5 5 22 LiCoO₂Art. LiPF₆ EC/PC/VC DD.pivalate 0.99 90.1 1M 45/45/5 5 8 LiCoO₂ Art.LiPF₆ EC/PC/VC None 0.45 15.2 (Com.) 1M 47.5/47.5/5 Remarks: No linearcarbonate was contained in any of Examples 16 to 22 and ComparisonExample 8 Initial: Initial discharge capacity

From the results set forth in Table 2, it is understood that a lithiumsecondary battery employing a positive electrode of lithium compoundoxide material, a negative electrode of carbonaceous material, and anelectrolytic salt of a fluorine-containing lithium salt shows a highdischarge capacity retention when such a small amount as 35 weight % orless of a tertiary carboxylic ester of the formula (I), particularlypivalic ester, is added to a non-aqueous solvent comprising a cycliccarbonate. It is further understood that generally the initial dischargecapacity greatly decreases when no linear carbonate is added to thenon-aqueous solvent (Comparison Example 8), while the decrease ofinitial discharge capacity prominently lowers and the discharge capacityretention apparently increases when a small amount of a tertiarycarboxylic ester of the formula (I), particularly pivalic ester, isadded in place of the linear carbonate.

It is furthermore noted that the initial discharge capacity reaches alevel equal to that shown in the case of using a linear carbonate and ahigh discharge capacity retention is observed when the alcohol residue(R⁴) of the tertiary carboxylic ester of the formula (I) is an alkylgroup having 4 or more carbon atoms.

EXAMPLE 22

Permeability of an electrolytic solution into pores of a microporousseparator to be used in a lithium secondary battery was evaluated in thebelow-mentioned manner.

An electrolytic solution was prepared by adding 2 or 4 weight parts of apivalic ester to 100 weight parts of 1M LiPF₆/PC electrolytic saltsolution. In the electrolytic solution was immersed for a period of 20seconds a separator of polypropylene microporous film (trademark:CELGARD #2500, available from CELGARD Inc.). The separator was thentaken out and its light permeability was observed. The results are setforth in Table 3.

TABLE 3 Amount Ethyl Butyl Hexyl Octyl Dodecyl (parts) pivalate pivalatepivalate pivalate pivalate 2 opaque S-T. A.T. F.T. F.T. 4 S-T. A.T. F.T.F.T. F.T. Remarks: S-T.: semitransparent A.T.: almost transparent F.T.:fully transparentFrom the results in Table 3, it is understood that a pivalic ester whosealcohol residue is an alkyl group having 4 or more carbon atoms showscompatibility with a separator higher than that shown by ethyl pivalatewhose alcohol residue is an alkyl group having 2 carbon atoms and, whenit is placed in contact with a microporous separator, rapidly permeatesinto a porous structure of the separator. This means that the period oftime for manufacturing a lithium secondary battery can be shortened. Inmore detail, the lithium secondary battery is manufactured by mounting acomposite of a positive electrode sheet, a separator, and a negativeelectrode sheet in a battery case, placing an electrolytic solution inthe case, and fixing a cap aver the case. The fixation of a cap over thecase ought to be done after the electrolytic solution completelyreplaces air occupying the micro-porous structure of the separator.Accordingly, the use of an electrolytic solution which is able toquickly permeate into a microporous structure of a separator shortensthe period of time for manufacturing lithium secondary batteries.

The present invention is not limited to the working examples describedherein, and various combinations apparent from the gist of the inventioncan be employed. Particularly, the invention is not limited to thecombinations of solvents shown in the working examples. Further, itshould be noted that although the above-stated working examples are fora coin battery, the present invention can be employed for cylindricaland prismatic batteries and polymer batteries.

UTILIZATION IN INDUSTRY

The use of a nonaqueous electrolytic solution of the invention in themanufacture of a lithium secondary battery provides a lithium secondarybattery that is excellent in battery characteristics such as its batterycycle property, electric capacity and stability endurance under chargedcondition, and further is essentially free from its expansion in the useat elevated temperatures.

1. A lithium secondary battery which comprises a positive electrodecomprising lithium compound oxide, a negative electrode comprisingcarbonaceous material, a separator, and a non-aqueous electrolyticsolution comprising an electrolytic salt in a non-aqueous solvent, inwhich the electrolytic salt is a fluorine-containing lithium salt andthe non-aqueous solvent contains a cyclic carbonate and 0.5 to 35 weight% of a tertiary carboxylic ester having the formula (I):

wherein each of R¹, R² and R³ independently represents methyl, ethyl, orfluorine, and R⁴ represents a hydrocarbyl group having 1-20 carbon atomsselected from the group consisting of an alkyl group, vinyl, allyl,propargyl, aryl group, and benzyl.
 2. The lithium secondary battery ofclaim 1, in which the electrolytic salt is selected from the groupconsisting of LiPF₆, LiBF₄, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiC(SO₂CF₃)₃,LiPF₄(CF₃)₂, LiPF₃(C₂F₅)₃, LiPF₃(CF₃)₃, LiPF₃(iso-C₃F₇)₃, andLiPF₅(iso-C₃F₇).
 3. The lithium secondary battery of claim 1, in whichthe cyclic carbonate is contained in the non-aqueous solvent in anamount of 10 to 80 weight %.
 4. The lithium secondary battery of claim1, in which the cyclic carbonate is selected from the group consistingof ethylene carbonate, propylene carbonate, butylene carbonate andvinylene carbonate.
 5. The lithium secondary battery of claim 1, inwhich the non-aqueous solvent further contains a linear carbonate in anamount of 80 weight % or less.
 6. The lithium secondary battery of claim1, in which the tertiary carboxylic ester is contained in thenon-aqueous solvent in an amount of 1 to 20 weight %.
 7. The lithiumsecondary battery of claim 1, in which the negative electrode comprisesnatural or artificial graphite.
 8. The lithium secondary battery ofclaim 1, in which R⁴ of the formula (I) is an alkyl group having 4-20carbon atoms.
 9. A lithium secondary battery which comprises a positiveelectrode comprising lithium compound oxide, a negative electrodecomprising carbonaceous material, a separator, and a non-aqueouselectrolytic solution comprising an electrolytic salt in a non-aqueoussolvent, in which the electrolytic salt is a fluorine-containing lithiumsalt and the non-aqueous solvent contains a cyclic carbonate and 0.5weight % or more of a tertiary carboxylic ester having the formula (I):

wherein each of R¹, R² and R³ independently represents methyl, ethyl, orfluorine, and R⁴ represents a hydrocarbyl group having 1-20 carbon atomsselected from the group consisting of an alkyl group, vinyl, allyl,propargyl, aryl group, and benzyl.
 10. A non-aqueous electrolyticsolution for lithium secondary batteries comprising an electrolytic saltin a non-aqueous solvent, in which the electrolytic salt is afluorine-containing lithium salt and the non-aqueous solvent contains acyclic carbonate and 0.5 to 35 weight % of a tertiary carboxylic esterhaving the formula (I):

wherein each of R¹, R² and R³ independently represents methyl, ethyl, orfluorine, and R⁴ represents a hydrocarbyl group having 1-20 carbon atomsselected from the group consisting of an alkyl group, vinyl, allyl,propargyl, an aryl group, and benzyl.
 11. The non-aqueous electrolyticsolution of claim 10, in which the electrolytic salt is selected fromthe group consisting of LiPF₆, LiBF₄, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂,LiC(SO₂CF₃)₃, LiPF₄(CF₃)₂, LiPF₃(C₂F₅)₃, LiPF₃(CF₃)₃, LiPF₃(iso-C₃F₇)₃,and LiPF₅(iso-C₃F₇).
 12. The non-aqueous electrolytic solution of claim10, in which the cyclic carbonate is contained in the non-aqueoussolvent in an amount of 10 to 80 weight %.
 13. The non-aqueouselectrolytic solution of claim 10, in which the cyclic carbonate isselected from the group consisting of ethylene carbonate, propylenecarbonate, butylene carbonate and vinylene carbonate.
 14. Thenon-aqueous electrolytic solution of claim 10, in which the non-aqueoussolvent further contains a linear carbonate in an amount of 80 weight %or less.
 15. The non-aqueous electrolytic solution of claim 10, in whichthe tertiary carboxylic ester is contained in the non-aqueous solvent inan amount of 1 to 20 weight %.
 16. The non-aqueous electrolytic solutionof claim 10, in which R⁴ of the formula (I) is an alkyl a hydrocarbylgroup having 4-20 carbon atoms.
 17. A non-aqueous electrolytic solutioncomprising an electrolytic salt in a non-aqueous solvent, in which theelectrolytic salt is a fluorine-containing lithium salt and thenon-aqueous solvent contains a cyclic carbonate and 0.5 weight % or moreof a tertiary carboxylic ester having the formula (I):

wherein each of R¹, R² and R³ independently represents methyl, ethyl, orfluorine, and R⁴ represents a hydrocarbyl group having 1-20 carbon atomsselected from the group, consisting of an alkyl group, vinyl, allyl,propargyl, aryl group, and benzyl.